The aryl hydrocarbon receptor (AHR) and AHR nuclear translocator (ARNT) are DNA binding transcription factors with basic helix-loop-helix/Per-Arnt-Sim (bHLH-PAS) domains. These two proteins form a heterodimer that mediates the toxic and biological effects of the environmental contaminant and AHR ligand 2,3,7,8-tetrachlorodibenzo-p-dioxin. The coiled-coil protein coiled-coil coactivator (Co-CoA) is a secondary coactivator for nuclear receptors and enhances nuclear receptor function by interacting with the bHLH-PAS domain of p160 coactivators. We report here that CoCoA also binds the bHLH-PAS domains of AHR and ARNT and functions as a potent primary coactivator for them; i.e. CoCoA does not require p160 coactivators for binding to and serving as a coactivator for AHR and ARNT. Endogenous CoCoA was recruited to a natural AHR target gene promoter in a 2,3,7,8-tetrachlorodibenzo-p-dioxin -dependent manner. Moreover, reduction of CoCoA mRNA levels by small interfering RNA inhibited the transcriptional activation by AHR and ARNT. Our data support a physiological role for CoCoA as a transcriptional coactivator in AHR/ARNT-mediated transcription.